This invention relates to papermaker's fabrics and in particular to fabrics, generally known as wet press felts or wet felts which are used in the wet press section of a papermaking machine.
Papermaker's wet felts are designed to transport an aqueous-partially formed web of paper through the press rollers in the wet press section of a papermaking machine and to assist in the further dewatering thereof. In its most common form, a papermaker's wet felt is constructed from a woven base fabric having fibrous batts needled to one or both sides.
The amount of void volume within the base fabric of a wet felt and its dewatering ability is directly related to the amount of water which can be internally handled by the felt construction while in the press nip. In other words, felts which can run with a "dry nip" (no water puddling behind the nip) are less likely to result in crushing or other hydraulic phenomena which are known to those skilled in the art.
It has been recognized in the art that it is possible to maintain a desired controlled void volume within the fabric construction by employing multi-layered base fabrics. One example of such a felt is disclosed in U.S. Pat. No. 4,356,225 which is assigned the assignee of the present invention. The fabric of U.S. Pat. No. 4,356,225 is disclosed as having increased stability while maintaining the machine direction layers in vertical alignment. Other examples are also cited in that patent.
The surfaces of the base fabric of conventional fabrics are predominantly defined by the top and bottom layers of machine direction yarns. The cross machine direction yarns which interweave the multiple layers of machine direction yarns of such fabrics protrude beyond the surfaces of the base fabric with sharp infrequent knuckles. It has been discovered that under the intense pressure of the nip, water removal can be impaired by the extremes of high and low pressure caused by the cross machine direction yarn knuckles on the paper bearing side of the fabric. Also, on the other or machine side of the fabric, the knuckles represent high pressure points which result in accelerated wear of the fabric.
It has been recognized in the art that it is possible to achieve improved pressing service and machine surface contact while maintaining controlled void volume within the fabric construction of multi-layered base fabrics. One example of such a felt is disclosed in U.S. Pat. No. 4,461,803 which is assigned to the assignee of the present invention. The fabric of the U.S. Pat. No. 4,461,803 patent comprises a multi-layered base fabric having both a smooth pressing surface and a roller surface resistant to wear in which the cross machine direction yarns define the predominate surfaces of the base fabric without creating sharp knuckles on either surface of the base fabric. However, in U.S. Pat. No. 4,461,803 construction the binder yarns extend between the top layer and the bottom layer of the fabric as machine direction yarns extend under or above the respective layer, and do not contribute to the pressing pressure points of the water removal surface, the top layer, or the machine roller contact surface, the bottom layer.
It has been recognized in the art that it is possible to utilize two separate fabric layers and to join the two fabric layers by means of an independent binder or a binder system comprised of threads from one of the fabric layers. One example of such a felt is disclosed in U.S. Pat. No. 3,214,326. The fabric of U.S. Pat. No. 3,214,326 which utilizes binder threads from one of the fabric layers results in the binder threads reducing the sheet contact surface area rather than in increasing the sheet surface contact area. The U.S. Pat. No. 3,214,326 construction using individual binder threads need not result in such a reduction of the sheet contact area, however, the binder threads do not contribute to the sheet surface contact area.